(1) Field of the Invention
The present invention is directed to the curing or vulcanizing of elastomers with free radical generating compounds, commonly referred to as vulcanization initiators. More particularly, the present invention relates to curable elastomer compositions in which vulcanization of the rubber is delayed by incorporating a certain methyl-substituted aminoalkyl phenol, as an inhibitor or a scorch retarder in the rubber composition to be vulcanized. The incorporation of the retarders of the present invention eliminates the use of potential carcinogenic materials or sublimeable lachrymators conventionally employed as retarders in this art. In addition, the prevulcanization inhibitors employed in the present invention exhibit better compatibility in most synthetic rubber systems, and further reduce or eliminate stain-producing residuals commonly experienced with use of Nnitroso diphenylamine retarders, conventionally employed for retardation of such elastomer compositions. Included in these systems are polyfunctional acrylic or methacrylic monomers, allylic compounds or metal salts of .alpha.,.beta. unsaturated carboxylic acids which are incorporated into the free radical curable elastomer compositions to be retarded and which provide improved technical advantages as reactive coagents in the crosslinking of such elastomeric compositions.
(2) Description of the Prior Art
The Vanderbilt Rubber Handbook (1978) discloses the use of a class of compounding materials designated as scorch "retarders" in certain highly accelerated rubber stocks where unusually hot processing conditions are encountered (page 344). Illustrative examples of such materials include phthalic anhydride, salicylic acid and sodium acetate, which are listed as examples of acidic materials that act as retarders in sulfur vulcanization systems. N-nitrosodiphenylamine is also disclosed as a retarder effective in hot processing of natural and synthetic compounds. It is also disclosed that "REOGEN" also exerts some retarding effect on compounds besides aiding in processing without noticeably decreasing accelerator activity on the curing conditions. In addition, the Vanderbilt text indicates that it has become standard practice to include fatty acids, such as stearic acid, in all natural rubber compounds to modify the curing rate of natural rubbers. All of these compounding materials, in general, are described as adaptable for use in sulfur vulcanization of rubber.
Parkes, U.S. Pat. No. 3,384,613, issued May 21, 1968, describes the use of nitroaryl, nitrosoaryl and N-nitroso-diaryl amine compounds as suitable modifiers in the retardation of scorch in rubber compounds containing di-tertiary peroxides, such as dicumyl peroxide, as the vulcanizing agent. Furthermore, Cowperthwaite et al., U.S. Pat. No. 3,751,378 issued Aug. 7, 1973, discloses the use of certain N-nitrosodiphenylamines and N,N'-dinitroso para-phenol diphenylamine compounds as retarders in the prevulcanization of rubber with polyfunctional methacrylate monomers as crosslinking agents with peroxides. However, although the industry has, in the past, found the N-nitroso-aryl compounds to be practical, the advent of TOSCA and EPA examination of industrial chemicals has found that such N-nitrosoamines to be especially toxic and often carcinogenic. Another disadvantage characteristic of the N-nitroso compound retarders of the prior art is their failure to totally co-react into acrylic monomer/elastomer systems, thereby producing residual staining of the surface of the resultant rubber article.
Furthermore, the Chemical Marketing Reporter (Jan. 2, 1984) pages 4 and 21, reports that nitrosoamines have been identified as a cause of cancer, and as a result thereof, the use of such compounds in the production of baby bottle nipples and related rubber articles of manufacture has been severely limited. Such findings have caused alarm in the rubber industry and instituted a search of similar functional compounds which are capable of retarding the onset of vulcanization, while at the same time, avoiding adverse effects on the final physical, mechanical properties of the cured rubber.
In the search for such compounds relating to the present invention, the present inventors have uncovered a number of compounds such as p-benzoquinone, 1,4-naphthoquinone, 9,10anthroquinone, their aliphatic substituted analogs, as well as their dioxime derivatives, and cupferon, each of which are either known carcinogens or sublimable lachrymators, thereby rendering these materials dangerous to work with in the usual rubber compounding work environments. Finally, Japanese Pat. No. 75-151239 dated Dec. 4, 1975 discloses the use of certain amine salts of 3,5-dinitrobenzoic acid and 2,6-substituted-4-dimethylaminomethyl phenols as vulcanization accelerators for neoprene rubber in the conventional sulfur vulcanization rubber process.